To find out when the workstation was previously locked look backwards in time for for event ID 4800.

If a screen saver is used, there is also a relationship between this event and 4802 (screen saver invoked) and 4803 (screen saver dismissed). For Interactive logons you may see this event or 4803. See event ID 4802 for the sequence of events.

Free Security Log Resources by Randy

Subject:

The user and logon session involved.

Security ID: The SID of the account.

Account Name: The account logon name.

Account Domain: The domain or - in the case of local accounts - computer name.

Logon ID is a semi-unique (unique between reboots) number that identifies the logon session. Logon ID allows you to correlate backwards to the logon event (4624) as well as with other events logged during the same logon session.